* add Dash-specific paths to `folders` in optimize-pngs.py
* `mogrify src/qt/res/*/*/*.png`
should fix "libpng error: IDAT: chunk data is too large"
* `python contrib/devtools/optimize-pngs.py`
"Total reduction: 500275 bytes"
- Check that image contents match pre- and post- crushing.
- Also remove use of external tool to compute sha256 in favor of hashlib.
- contrib: remove all use of shell=True in strip_pngs.py
Using `shell=True` can be a security hazard. See e.g.
https://docs.python.org/2/library/subprocess.html#subprocess.check_output
